13C17O suggests gravitational instability in the HL Tau disc

We present the first detection of the 13C17O J = 3–2 transition toward the HL Tau protoplanetary disc. We find significantly more gas mass (at least a factor of 10 higher) than has been previously reported using C18O emission. This brings the observed total disc mass to 0.2 M⊙, which we consider to be a conservative lower limit. Our analysis of the Toomre Q profile suggests that this brings the disc into the regime of gravitational instability. The radial region of instability (50–110 au) coincides with the location of a proposed planet-carved gap in the dust disc, and a spiral in the gas. We therefore propose that if the origin of the gap is confirmed to be due to a forming giant planet, then it is likely to have formed via the gravitational fragmentation of the protoplanetary disc.